I personally think it's two reasons:
1. Poor matchups. They love putting the NFC East team in there to draw the most ratings but more often than not they're bad. Looking at the remaining MNF games really the only game I really want to watch is Falcons-Seahawks. Where Sunday Night Football has: Pats-Falcons, Pats-Broncos, Eagles-Seahawks, Ravens-Bucs with the ability to flex any game to make it better later. The NFL should look into flexing MNF too
2. Poor commentating. TNF quickly realized Tony Romo was great to have in the booth for TNF..... Gruden is boring; likes everyone and doesn't show me anything. McDonough?